MONTCOAL, W.Va. -- A federal safety official said more air testing would be needed before rescue teams could head back into a West Virginia coal mine Thursday night to look for four miners missing since an explosion killed 25 workers.

Kevin Stricklin, coal administrator from the federal Mine Safety and Health Administration, said levels of noxious gases had dropped but not enough for the crews to re-enter the Upper Big Branch mine about 30 miles south of Charleston, W.Va.

He said sampling would continue. If levels failed to drop by ventilation, crews were planning to pump in nitrogen to neutralize the atmosphere. It was unclear why they didn't start pumping in nitrogen sooner.

Rescuers were eager to resume the underground search in the slim hope that the men made it to one of the rescue chambers in the mine.

"We committed to the families we were going to get into the chambers within 96 hours and we're doing everything in our power to do that," Stricklin said.

Earlier in the day, searchers came within 500 feet of a rescue chamber where possible survivors may have taken refuge, but were told to abandon their mission because of the explosive mix of gases had become too dangerous.

Teams spent more than four hours in the morning working their way by rail car and on foot through the mine where 25 workers were killed Monday in the worst U.S. mining disaster in more than two decades. When told to abandon their mission, team members were angry, but their safety was paramount, said Chris Adkins, chief operating officer for mine owner Massey Energy Co.

Crews at the surface resumed drilling to get fresh air into the mine. Gov. Joe Manchin said Thursday evening that the levels were near those considered safe.

"We're just moving as quickly as we can," Manchin said. "We want to bring the loved ones back."

Rescue teams were headed first to the airtight chamber that has at least four days worth of food, water and oxygen.

Massey's chief executive officer, Don Blankenship, continued to defend his company's record and disputed accusations from miners that he puts coal profits ahead of safety.

"To some extent the fact that there were more survivors than those that are lost suggests that the mine was in pretty good shape relative to what mines would have been in the past and hopefully by today's standards," he told The Associated Press in an interview Thursday. There were 61 miners in Upper Big Branch when it was rocked by the blast.

Despite the increasingly slim chance of finding anyone alive, Adkins said he considered Thursday's effort a rescue mission.

"I still believe in God, I believe, and I'm not going to give up," he said.